The present disclosure relates to image forming apparatuses such as copiers, printers, facsimile machines, etc., and in particular, relates to a method for stabilizing a recording-medium conveyance state on the upstream side of an image carrier and a transfer member.
In an image forming apparatus using an electrophotography method, a toner image is formed by making toner adhere to an electrostatic latent image formed on an image carrier such as a photosensitive drum or the like, the toner image is transferred onto a sheet as a recording medium, such as a sheet of paper or the like, and then the toner image on the recording medium is fixed by a fixing device (fixing section).
In such an image forming apparatus, units such as a photosensitive drum, a developing device, and the like are densely arranged for compactness, making airtightness high around the photosensitive drum. In the configuration described above, for example, at a time when the leading edge of the sheet enters, from a registration roller pair, into a nip (transfer nip) formed between a photosensitive drum and a transfer roller, or at a time when the rear edge of the sheet leaves the nip of the registration roller pair or an intermediate roller, if the state of sheet conveyance changes (for example, fluttering or abrupt position change of the sheet), the change causes a change in volume of a conveyance space, and airflow is generated by a change in air pressure ascribable to the volume change.
When passing through a gap (development nip) between the photosensitive drum and a developing roller, this airflow scatters toner particles having been caused by a developing electric field to fly from a developing roller to the photosensitive drum. As a result, the scattered toner particles sometimes adhere to the photosensitive drum at wrong positions deviated from their appropriate adhesion positions, generating horizontal stripes in a halftone image or in a solid image.
On the other hand, if the gap is reduced between upper and lower pre-transfer guides arranged on the upstream side of the transfer nip with respect to a sheet conveyance direction to thereby reduce the fluttering of the sheet, it may increase a conveyance load in conveying a hard paper sheet such as a thick paper sheet, to cause transfer defect such as reduction in transfer magnification, transfer misalignment, etc.
As a solution to such inconvenience, there has been proposed a method for reducing defective transfer by smoothly guiding a sheet into the transfer nip both in the case of using a regular paper sheet and in the case of using a thick paper sheet, and there have been known a process cartridge and an image forming apparatus, for example, in which first and second guide members constituted by flexible film members are provided on the upstream side of a transfer position to thereby maintain a regular paper sheet in an appropriate state for entering the photosensitive body on one hand and to reduce the load with respect to a thick paper sheet on the other hand.
There is also known a configuration that includes a flexible guide plate which supports, on one of its surfaces, a sheet conveyed by a sheet feed roller or the like, and guides the sheet toward a photosensitive drum, and a sponge that is provided on the other surface of the guide plate and is softer than the guide plate, such that the sponge is formed to project toward the photosensitive drum beyond the photosensitive-drum side edge of the guide plate, whereby vibration of the guide plate and the fluttering of the rear end of the sheet are reduced, and flapping sound made by the guide plate is also reduced.